Magnetic drive mechanisms for printing heads

ABSTRACT

There is disclosed a drive mechanism for a printing head of a printing machine of the kind where the print head moves with respect to the material to be printed, the mechanism including an elongated electromagnet to provide a magnetic field, a coiled electrical conductor slidably mounted for movement under the influence of the magnetic field and adapted for connection to a print head, and a circuit connected to the conductor for sensing the position of the electrical conductor with respect to a datum and for controlling the supply of electric current to the conductor so as to control the movement of the conductor. The supply of current to the electromagnet and the coil are independently variable. The position sensing circuit can be used to control the supply of current to the electromagnet and the coil.

madam United States Patent 1191 Kitz et al,

1111 3,867,675 Feb. 18, 1975 1541 MAGNETIC DRIVE MECHANISMS FOR PRINTINGHEADS a [75] Inventors: Norbert Kitz; James John Drage,

both of Uxbridge,-England [73] Assignee: Bell Punch Company Limited,

Uxbridge, England [22] Filed: Aug. 13, 1973 [21] Appl. No.: 388,664

[30] Foreign Application Priority Data Aug. 18, 1972 Great Britain38759/72 Nov. 17, 1972 Great Britain 53193/72 [52] US. Cl 318/119,318/127, 318/135, 310/l4,3l0/15 [51] Int. Cl l H02k 33/00 .HO2k41/00[58] Field of Search 318/119, 122, 126, 127,

[56] References Cited UNITED STATES PATENTS 3,688,035 8/1972 Cless .1310/13 X Primary Examiner-Gerald Goldberg Attorney, Agent, orFirm-Laurence R. Brown [5 7] ABSTRACT There is disclosed a drivemechanism for a printing head of a printing machine of the kind wherethe print head moves with respect to the material to be printed, themechanism including an elongated electromagnet to provide a magneticfield, a coiled electrical conductor slidably mounted for movement underthe influence of the magnetic field and adapted for connection to aprint head, and a circuit connected to the conductor for sensing theposition of the electrical conductor with respect to a datum and forcontrolling the supply I of electric current to the conductor so as tocontrol the movement of the conductor.. The supply of current to theelectromagnet and the coil are independently variable. The positionsensing circuit can be used to control the supply of current to theelectromagnet and p the coil.

5 Claims, 12 Drawing Figures Pmmgmimsmrs 3,867,675

CONTROL INPUTS OUTPUTS FROM 5 22 5 T0 POSITION SENSORS 2' Hm I PRINTHEAD 46 44 42 W 47 /PRINT 1 MAGNETIC DRIVE MECHANISMS FOR PRINTING HEADSThis invention relates to printing machinesof the kind where onoperation a print head is reciprocated with respect to the materialwhich is printed by the print head. The definition ofprint head includesheads which print by the impact of movable members, for example hammersand solenoid driven needles, heads which print by heat, light and otherforms of electromagnetic waves, heads which print by the projection orattraction of charged particles to mark the material and any other formof head by which material is marked.

The definition of print head includes a head using a combination of anyof the above material-marking methods. Examples of such printingmachines are typewriters which use the gold ball print head and thosedigital printers, such as line printers in which the print head moveswith respect to the material to be printed. it has been previously knownto produce the relative motion by means of a drive mechanism having anelectric drive motor geared to an endless belt to which is attached tothe print head. The drive motor is controlled by mechanically actuatedcontrol circuits to control the extent of movement in one direction andreversal of movement after movement of a predetermined extent. The knowndrive mechanism has the disadvantages of complexity, high cost, andspeed limitations because of the inertia forces in the mechanism. It hasbeen proposed to produce the relative motion by means of a bar ofhigh-permeability magnetic material which passes through a solenoidwhich is connected to a print head. Thisproposal suffers from thedisadvantages that the print head is limited to a low weightbecause oflevel of flux density of the magnetic field; which flux density cannotbe increased to accelerate a standard print head and solenoid so as toovercome the inertia and static friction forces to give a speedcomparable with prior drive mechanisms. Furthermore, the bar of magneticmaterial is heavy and is expensive in the amount required to produce thestandard carriage movement or stroke.

According to the present invention there is provided a drive mechanismfor a print head of a printing machine, the mechanism including amagnetic circuit having an elongated member and having at least oneelectromagnet circuit to provide a magnetic field normal to thelongitudinal axis of the elongated member, an electrical conductorslidably mounted relative to the elongated member for movement under theinfluence of the magnetic field and adapted for connection to a printhead, an electric circuit for supplying electric current to theconductor, and switch means connected in at least one of the circuits soas to control the extent of movement of the conductor.

Preferably the electric circuit for supplying electric current to theconductor includes means for controlling the variation of the electriccurrent supplied.

According to a further aspect of the present invention there is provideda drive mechanism for a print head of a printing machine, the mechanismincluding an electromagnet having a first elongated member and at leasta second elongated member connected parallel with the first member toprovide a magnetic field between the members, an electrical conductorslidably mounted on a member for movement under the influence of themagnetic field and adapted for connection to a print head, and a circuitconnected to the conductor for controlling the supply of electriccurrent thereto so as'to control the extent of movement of theconductor. V

Preferablythe electrical conductor is a solenoid "of at least one helixwrapped aroundan elongated member.

Preferably the electromagnet; includes a circuit .for controlling thevariation of the electric current supplied to the electro-magnet.

Preferably at least one of the circuits supplying electric current tothe electric conductor and to the electromagnet includes means forchanging the polarity of the electric current supplied.

Preferably the control circuit includes a means for sensing the positionof the electrical conductor with respect to a datum.

According to a still further aspect of the present invention there isprovided a drive mechanism for a print head of a printing machine, themechanism including means to provide a magnetic field, an electricalconductor slidably mounted for movement under the influence of themagnetic field and adapted for connection to a print head and a circuitconnected to the conductor for sensing the position of the electricalconductor with respect to a datum and for controlling the supply ofelectric current to the conductor so as to control the movement of theconductor.

Preferably the electrical conductor is a solenoid of at least one helixwrapped around an elongated member.

Preferably the magnetic-field-providing means is an electromagnet; andpreferably the control circuit includes a circuit for controlling thevariation of the electric current supplied to the electro-magnet.

Preferably the electric current supplied to the electro-magnet circuitcan be varied independently of the current supplied to the electricalconductor.

Preferably at least one of the circuits supplying elec' tric current tothe electrical conductor and the electromagnet includes means forchanging the polarity of the electric current supplied.

Preferably the circuit for sensing the position of the electricalconductor causes the control circuit to control the supply of electricalcurrent to the electrical conductor and to the electromagnet so as tocontrol the movement of the conductor.

Preferably the electrical conductor can be'automatically returned to aninitial position from any point on the extent of movement of theconductor.

Preferably the electrical conductor can move by at least one characterposition after an extent of movement equal to at least one character tofacilitate reading what has been printed so far.

Preferably the electrical conductor can move an extent equal to at leastone character position to print at least one character, can reverse themovement by at least the same number of character positions tofacilitate reading what has been printed so far and can then reverse tomove in the original direction to print at least one further character.

Constructional embodiments will now be described, by way of examplesonly, with reference to the accompanying drawings wherein:

FIG. 1 shows an isometric view of a first drive mechanism made inaccordance with the invention;

FIG. 2 shows a plan cross-section taken on the line A-A on the drivemechanism shown in FIG. 1 together with a diagram of the circuits whichcontrol the operation of the drive mechanism;

FIG. 3 shows a vertical cross-section taken on the line 8-8 or the drivemechanism of FIG. 1;

FIG. 4 shows an isometric view of a second drive mechanism made inaccordance with the invention;

FIG. 5 shows a vertical cross-section taken on line A-A on the drivemechanism shown in FIG. 4 together with a diagram of the circuits whichcontrol the operation of the drive and the print head mechanism;

FIG. 6 shows a vertical cross-section taken on the line B-B on the drivemechanism of FIG. 4 with the side cover removed from the positionsensor;

FIG. 7 shows a view of a third drive mechanism made in accordance withthe invention;

FIG. 8 shows a plan cross-section taken on the line A-A on the drivemechanism shown in FIG. 8 together with a diagram of the circuits whichcontrol the operation of the drive mechanism;

FIG. 9 shows a vertical cross-section taken on the line AA.on the drivemechanism of FIG. 8;

FIG. 10 shows an isometric view of a fourth drive mechanism made inaccordance with the invention;

FIG. 11 shows a vertical cross section taken on line AA on the drivemechanism shown in FIG. 10 together with a diagram of the circuits whichcontrol the operation of the drive and print head mechanism; and

FIG. 12 shows a vertical cross-section taken on line BB on the drivemechanism of FIG. 10 with the side cover removed from the positionsensor.

The first drive mechanism 10 shown in FIG. 1 has a base 12 whichconsists of a first elongated member in the form of a bar 14 ofrectangular cross-section and of magnetic material such as mild steel,second elongated members in the form in bars 16 of rectangularcrosssection and of magnetic material such as mild steel and a pair ofend plates 18 which connect the bar 14 to the bars 16. At each end ofthe base 12 a coil former 20 of non-magnetic material is secured on thebar 14 and a coil of wire 22 is wound on the coil former 20 to make thecarriage bar 14 act as an electromagnet.

As shown in FIG. 2 the coils 22 are connected to a smoothing circuit 24and to a full-wave rectifier bridge 26 which is connected to the mainsvoltage of 230 volts a.c. through a switch 28.

A coil former 30 having shock-absorbing material 34 on the cheeks of theformer is slidably mounted on the bar 14 and an electrical conductor inthe form ofa solenoid coil 32 is wound on the former 30. A standard seven-byfive matrix print head 36 is secured to the coil former 30 to clearthe upper edges of the bars 16. The

solenoid coil 32 is connected to the output of a control to positioncontrol circuitry (not shown) and the other inputs connected to theoutput of a full-wave rectifier bridge 42. The input of the rectifierbridge 42 is connected to the output of a step-down transformer 44 theoutput of which is connected through a switch 46 to the mains voltage of230 volts a.c.

In operation, the energised coils 22 generate a magnetic field betweenthe bar 14 and the bars 16 and this magnetic field urges the solenoidcoil 32, the former 30 and the print head 36 parallel with thelongitudinal axis of the bars 14 and 16 at a speed and a direction whichdepends on the magnitude and direction of current flow in the solenoidcoil 32 determined by the control circuit 40. The control circuit 40controls the speed of reciprocation of the print head 36 between aposition where the coil former 30 abuts one of the coil formers 20 and aposition where the coil former 30 abuts the other coil former 20 bygenerating a repeated symmetrical waveform in which the shape of thewaveform determines the speed and direction of the print head 36.

The control input 41 of the control circuit 40 is controlled by othercircuits (not shown) which drive the material to be printed (not shown)and the print head 36 so that seven successive reciprocations of theprint head 36 print a line of characters on the material.

The circuit connected to the coils 22 can be controlled instead of, oras well as, the circuit connected to the solenoid coil 32. The seconddrive mechanism 10, shown in FIG. 4, has the base 12 having the bars 14and 16, the end plates 18, the coil formers 20, the coils 22 aspreviously described and previously shown in FIGS. 1, 2 and 3.

As shown in FIG. 5 the coils 22 of FIG. 4 are connected to the smoothingcircuit 24 and to a full-wave rectifier bridge 26 which is connected tothe mains voltage of 230 volts a.c. through a switch 28.

A coil former 51 is slidably mounted on the uppermost bar of the bars 16and an electrical conductor in the form of a solenoid coil 48 is woundon the former 51. The former 51 is connected to an n-shaped saddle 50 ofnonmagnetic material which saddle is slidably mounted on the bar 14 bythe four pairs of rollers 52. The print head 36 is secured to the saddle50.

A tape 53 of rigid non-magnetic material is attached at each end to acheck of the coil former 20 respectively. The tape 53 passes through atape senser 54 which comprises a line of seven light-emitting diodes 56on one side of the tape 53 and a line of seven photoelectric diodes 58on the other side of the tape and parallel with the diodes 56. The tape53 is perforated longitudinally in seven lines to form a seven bit tapein which each signal of seven bits is a code representing the positionof this signal from a zero signal sensed when the saddle 50 abuts eachend of the coil formers 20 respectively. The connections to thelight-emitting diodes 56 and the photo-electric diodes 58 are connectedby a cable 60 to a control circuit 62. The control circuit 62 also hasconnections to the solenoid coil 48, and the print data inputconnections which transmit the codes representing the characters to beprinted and has connections to the outputs of the print head 36. Thecontrol circuit 62 also has a pair of inputs connected to the output ofthe full-wave rectifier bridge 42 which is connected to the output of atransformer 44 the input of which is connected through the switch 46 tothe main voltage of 230 volts a.c.

In operation the second drive mechanism operates similarly to the firstdrive mechanism. The print data transmitted to the control circuit 62includes control signals such as print on, print off, space, Tab, andcarriage return and these control signals are compared with the sevencontrol inputs from the tape sensor 54 to control the amplitude andpolarity of the dc. voltage transmitted to the solenoid coil 48 and theoperation of the print head 36.

The method of mounting the print head 36 of the second drive mechanism10 has the advantage that there is greater mechanical stability.

The print data transmitted to the control circuit 62 can be in any ofthe standard codes, such as the ASCll code.

The voltages transmitted from the control circuit 62 to the solenoidcoil 48 can vary in polarity and amplitude to control the direction andspeed respectively of the print head 36 and can have a pulsed duty cycleand pulsed amplitude in response to tabulate or carriage return signals.

The circuits connected to the solenoid coils 22 can be controlledinstead of, or as well as, the circuit connected to the solenoid coil48.

The third drive mechanism shown in FIGS. 7 to 9 has a rod 11 of magneticmaterial such as mild steel, a tube 14' of magnetic material such asmild steel with an axial slot 16 (FIG. 1) in the tube 14' and a pair ofcircular end plates 18 which are of magnetic material such as mild steeland which connect the tube 14' coaxially to the permanent magnet rod 11.On the rod 11 and adjacent the end plates 18 are circular coil formers20 each with a coil of wire 22 which is connected to the smoothingcircuit 24, the rectifier bridge 26 and the switch 28 previouslydescribed with reference to FIGS. 2 and 5 so that the rod 11 forms anelectromagnet.

A coil former 30 is slidably mounted on the rod 11 and an electricalconductor in the form of a solenoid coil 32 is wound on the former 30. Astandard sevenvertical matrix print head 36 is secured by a pair of L-shaped brackets 33 made of non-magnetic material to the coil former 30to clear the upper edges of the axial slot 16' in the tube 14'. A pairof non-magnetic roller bearings 34 (FIG. 3) are mounted between theprint head 36 and the tube 14'. The solenoid coil 32 is connected to theoutput of a control circuit 37 having inputs 38 which are connected tosolenoid-positionsensing circuitry (not shown) which senses the positionof the coil former 30 with respect to a datum position so as to controlthe supply ofelectric current to the solenoid coil 32. The controlcircuit 37 has outputs 39 which are connected to the print head 36 andinputs 40 which are connected to a source of print data. The otherinputs 41 of the control circuit 37 are connected to the output of afull-wave rectifier bridge 42. The input of the rectifier bridge 42 isconnected to the output of a stepdown transformer 44 the input of whichis connected through a switch 45 to the mains voltage 230 volts a.c.

In operation, the magnetised rod 11 generates a magnetic field betweenthe rod 12 and the tube 14' and this magnetic field urges the solenoidcoil 32, the former 30 and the print head 36 parallel with the coaxiallongitudinal axes of the rod 12 and the tube 14 at a speed and adirection which depends on the magnitude and direction of current flowin the solenoid coil 32 determined by the solenoid-position sensingcircuitry (not shown) and the control circuit 37. The control circuit 37controls the speed of reciprocation of the print head 36 between aposition where the coil former 3,0 abuts one of the coil formers 20 anda position where the coil former 30 abuts the other coil former 20 bygenerating a repeated symmetrical waveform in which the shape of thewave-form determines the speed and direction of the print head. Thecontrol circuit 37 automatically controls the supply of current to thesolenoid coil 32 so that the print head 36 is automatically returned toan initial position which is at either of the farthest extents ofmovement of the print head along the axis of the rod 11. The initialpositions to which the print head 36 is automatically returned may be atpreselected positions along the axis of the rod 11. The control circuit37 is controlled by other circuits (not shown) with control the drive ofthe material to be printed (not shown) and the print head 36 so that atseven successive character positions the print head 36 prints afive-column character and leaves a space of two character positions onthe material. The control circuit 37 can control the supply of currentto the solenoid coil 32 so that the print head 36 is moved automaticallyby one character position after a movement equal to at least onecharacter position so as to facilitate reading what has been printed sofar. The control circuit 37 can also control print head 36 to moveautomatically by one character position. to reverse automatically tomove back by at least two character positions so as facilitate readingwhat has been printed so far, and then to reverse automatically to movein the original direction by at least three character positions.

A fourth drive mechanism 10' shown in FIG. 10 has a base 12 whichconsists of a first elongated member in the form of a bar 14 ofrectangular cross-section and of magnetic material such as mild steel,second elongated members in the form in bars 16 of rectangularcrosssection and of magnetic material such as mild steel, and a pair ofend plates 18 which connect the bar 14 to the bars 16. At each end ofthe base 12 a coil former 20 of non-magnetic material is secured on thebar 14 and a coil of wire 22 is wound of the coil former 20 to make thebase 12 act as an electromagnet.

As shown in FIG. 11 the coils 22 are connected to the smoothing circuit24 and to a full-wave rectifier bridge 26 which is connected to themains voltage of 230 volts a.c. through a switch 28.

A coil former 51 is slidably mounted on the bar 14 and an electricalconductor in the form of a solenoid coil 48 is wound on the former 51.The former 46 is connected to a U-shaped saddle 50 of non-magneticmaterial which saddle is slidably mounted on the bars 16 by the eightpairs of rollers 52. The print head 36 is secured to the saddle 50.

The fourth drive mechanism 10' includes solenoidpositioning circuitry inthe form of a rigid strip 53 of plastics material which is attached ateach end to a respective cheek of the coil former 20 and which carriestransverse lines (not shown) each line several thousandths of an inchwide and spaced such that the lined and non-lined portions of the strip53 are of equal width, and a casing 54 which is attached to the saddle50 and which has the strip 53 passing through. The casing 54 contains ashort strip 55 of plastics material which is attached to the casing 54with its longitudinal axis parallel with and spaced a few thousandths ofan inch from the longitudinal axis of the strip 53. The strip 55 carriesparallel lines (not shown) each equal in width and spacing to the lines(not shown) on the strip 53, but inclined by the width of half a line tothe normal of the longitudinal axis of the strip 55. The casing 54 alsocontains a lamp 56 and two phototransistors 57 and 58 arranged so thatthe strips 53 and 55 separate the lamp 56 from the phototransistors 57and 58. The phototransistors 57 and 58 are on opposite sides and areequispaced from the transverse axis of the strips 53 and 55. The lamp 56and the phototransistors 57 and 58 are connected by a cable 60 to acontrol circuit 62 which is similar to the control circuit 37. Thecontrol 7 circuit 62 also has connections to the solenoid coil 4-8 andthe print data input connections which transmit the codes representingthe characters to be printed and has connections to the outputs of theprint head 36. The control circuit 62 also has a pair of inputs 41connected to the output of the full-wave rectifier bridge 42 which isconnected to the output of the transformer 44 the input of which isconnected through the switch 45 to the mains voltage of 230 volts a.c.In operation the fourth drive mechanism operates similarly to the thirddrive mechanism. ln operation, the relative movement of the strip 55with respect to the strip 53 causes the generation of a succession ofmoire fringes which are parallel with longitudinal axes of the strips 53and 55 and which travel towards the longitudinal edges of the strips 53and 55 at a speed dependent upon the relative speeds of the strips 53and 55. The phototransistors 57 and 58 count the fringes to generatepulse trains in which the distance between each pulse is equal to onecolumn distance and in which the phase of the train of pulses from thephototransistor 57 with respect to the train of pulses from thephototransistor 58 (which train of pulses acts as a datum) is related tothe speed and direction of motion of the solenoid 48. The datum pulsesof the train of pulses from the phototransistor 58 are counted bycharacter counter circuit (not shown) in the control circuit 62, whichcharacter counter circuit transmits a start of track signal at a zeropulse count and in effect counts the number of column positions betweenthe position of the solenoid 48 and the start of track signal. Thepulses of the train of pulses from the phototransistor 58 are alsodivided into successive groups of seven pulses which pulses controlcircuits (not shown) within the control circuit 62 so that the firstfive pulses allow the printing of five columns by the print head 36 isone complete character, and the other two pulses do not allow printingso that there is a space of two columns between successive characters.

The speed of the solenoid 48 is maintained approximately constant by theaction of speed control pulses formed by differentiating the trains ofpulses from the phototransistor 57 and 58 upon the circuit (not shown)in the control circuit 62 which supplies current 62 which suppliescurrent pulses to the solenoid 48. The differentiated datum pulses fromthe phototransistor 38 control the end of the solenoid current pulsesand the differentiated pulses from the phototransistor 57 override thenormal beginning of the solenoid current pulses at approximately 700usec from the end of the current pulses so as to control the width ofthe solenoid current pulses. The control action is a form of negativefeedback in that as the solenoid speed increases the width of thesolenoid current pulses decreases to supply less current to the solenoid48 and when the solenoid 48 slows down, the width of current pulsesincreases to increase the speed of the solenoid 48. The speed of thesolenoid 48 is thus maintained within two speed limits.

The print data transmitted to the control circuit 62 includes controlsignals such as print on, print off, space, tab and carriage return andthese control signals also control the amplitude and polarity of the dovoltage transmitted to the solenoid coil 48 and the operation of theprint head 36.

The method of mounting the print head 36 of the fourth drive mechanismhas the advantage that there is greater mechanical stability than themechanical stabilities of the first and second drive mechanisms.

The print data transmitted to the control circuit 62 can be in any ofthe standard codes such as the A511 code.

The voltages transmitted from the control circuit 62 5 to the solenoidcoil 48 can vary in polarity and amplitude to control the direction andspeed respectively of the print head 36 and can have a pulsed duty cycleand pulsed amplitude in response to tabulate or carriage return signals.The control circuit 62 can cause the automatic movement of the printhead 36 previously described in respect of the control circuit 37.

The circuits connected to the solenoid coils 22 can be controlledinstead of, or as well as. the circuit connected to the solenoid coil32.

The solenoid-position-sensing circuitry can include photo-opticaldevices magnetic devices or sliding contacts to generate pulsesrepresenting characters as column position.

The circuit for controlling the supply of currents to the electricalconductor can decelerate or stop the movement of electrical conductor byopen-circuiting or short-circuiting the electrical conductor or bysuddenly reversing the polarity of the current flowing through theelectrical conductor.

The circuit for controlling the supply of current to the electro-magnetcan decelerate or stop the move ment of the electrical conductor byreversing the polar ity of the current flowing through the electromagneteither separately or in combination with a change in the current flowingthrough the electrical conductor.

The circuits for controlling the supply of current to the electricalconductor and the electromagnet can operate separately or together tobring the electrical conductor to a stop without mechanical impact ateither end of its movement.

What we claim is:

l. A drive mechanism for moving a print head over a longitudinal pathcomprising in combination, a first elongated member in the form of alength of magnetic material such as mild steel disposed parallel to saidpath, a second elongated member in the form of a length of magneticmaterial such as mild steel disposed parallel to said path and spacedfrom said first member, means disposed for producing a magnetic fieldcoaxially through the length of one said member and with the two saidmembers located in a closed magnetic path for the magnetic field, and amovable print head carriage mounted on a selected one of said membersfor movement over said path comprising an electromagnetic coil disposedcoaxially around the selected mem' her for control of movement of saidcarriage along said path by means of a magnetic field generated therebyreacting with said magnetic field through the length of said member.

2. A drive mechanism as defined in claim 1 wherein said means producingthe magnetic field comprises at least one electromagnetic solenoiddisposed coaxially about one of said members.

3. A mechanism as defined in claim 1 including a longitudinal memberlocated parallel to and along said longitudinal path having definedtherein codes for identifying different positions along said path, andmeans for selecting said codes to move the print head to selectedpositions along said path.

4. A mechanism as defined in claim 1 including a longitudinal stripmember located parallel to and along said longitudinal path havingdefined thereon a set of being transparent, and wherein said meanscounting said lines comprises a set of phototransistors and a lamparranged to sense a succession of Moire fringes parallel to thelongitudinal axes of said two strip members.

1. A drive mechanism for moving a print head over a longitudinal pathcomprising in combination, a first elongated member in the form of alength of magnetic material such as mild steel disposed parallel to saidpath, a second elongated member in the form of a length of magneticmaterial such as mild steel disposed parallel to said path and spacedfrom said first member, means disposed for pRoducing a magnetic fieldcoaxially through the length of one said member and with the two saidmembers located in a closed magnetic path for the magnetic field, and amovable print head carriage mounted on a selected one of said membersfor movement over said path comprising an electromagnetic coil disposedcoaxially around the selected member for control of movement of saidcarriage along said path by means of a magnetic field generated therebyreacting with said magnetic field through the length of said member. 2.A drive mechanism as defined in claim 1 wherein said means producing themagnetic field comprises at least one electromagnetic solenoid disposedcoaxially about one of said members.
 3. A mechanism as defined in claim1 including a longitudinal member located parallel to and along saidlongitudinal path having defined therein codes for identifying differentpositions along said path, and means for selecting said codes to movethe print head to selected positions along said path.
 4. A mechanism asdefined in claim 1 including a longitudinal strip member locatedparallel to and along said longitudinal path having defined thereon aset of transverse lines and spaces of equal width, and means countingsaid lines as the print head moves along said longitudinal path.
 5. Amechanism as defined in claim 4 including a second longitudinal stripmember parallel to the first with a set of lines similar to that of thefirst, both said strips being transparent, and wherein said meanscounting said lines comprises a set of phototransistors and a lamparranged to sense a succession of Moire fringes parallel to thelongitudinal axes of said two strip members.